A program-controlled shutter for opening and closing the sectors of a camera shutter using a stepping motor has been proposed which avoids the use of a mechanical shutter charge mechanism and electrically and digitally performs exposure control.
To control the opening and closing of shutter sectors using a stepping motor, the stepping motor is first rotated in a forward direction to start opening the sectors, and then the sectors are stopped by rotating the stepping motor in a reverse direction. In such exposure control, the maximum open position of the shutter sectors is determined based on object brightness, film sensitivity and the like. If the determined maximum open position does not coincide with a unit step angle of the stepping motor, an exposure error occurs.
To eliminate such exposure error, a method as disclosed for example in Japanese Patent Laid-Open Publ. No. 60-57357 is known. In this method, the number of drive pulses is determined by an exposure value obtained through calculation, the drive pulses being used for driving the stepping motor and having a normal pulse width. In addition, an interpolating drive pulse is calculated for use in opening the shutter sectors to a proper or correct open position. Specifically, the stop position of the stepping motor during a forward rotation is determined by a plurality of drive pulses having a normal pulse width and an interpolating drive pulse having a narrower pulse width than normal.
With the above method, it is necessary to determine, prior to starting the stepping motor, the number of normal drive pulses and the pulse width of the interpolating drive pulse, which requires a complicated circuit configuration. Furthermore, the inertia of the drive transmission mechanism between the rotors or between the stepping motor and the shutter sectors, may cause a delay in the operation of the sectors. Because of this inertia, even if the stepping motor is rotated in a reverse direction at the instant the sectors reach the correct open position corresponding to the exposure value, the sectors will overrun the correct open position. Only thereafter will the sectors be moved back to the correct position. Such overrunning motion of the sectors is a serious drawback, particularly in the case of high shutter speed, and is in fact the main cause of over-exposure.